Coatings are used in a wide variety of applications to protect underlying structures from their environments. For example, coatings can be used to resist corrosion, to provide thermal insulation, to prevent mechanical damage, to reduce radar observability, or to protect from lightning strikes. Coatings include, for example, paint and polymer-based appliqués, which are being considered by military and commercial aviation operators as an alternative to paint.
For quality control, process monitoring, and cost control, it is desirable to have a method of non-destructive evaluation (NDE) of coating thickness during manufacturing. Preferably, the coating thickness could be accurately measured after it dries or while still wet, so that the coating application can be adjusted in real time to control the process. It is also desirable to be able to rapidly and accurately measure coating thicknesses in the field. Particularly in critical applications, such as to measure the thickness of hi-tech coatings used in aircraft and spacecraft, fast, accurate measurements are critical. Hi-tech coating, such as low observable coatings used in stealth aircraft, can be very expensive. An extra few thousands of an inch of unnecessary coating thickness can add significant expense to the manufacturing process for an aircraft part. Excess paint can also mean unnecessary added weight, and when a lighting strike layer is present, increased lightning hit probability.
One method used in the automotive industry to measure paint coating thickness entails the use of magnetic fields to determine the total thickness of paint on a steel substrate. This method is limited to measuring the thickness of coatings on metallic surfaces. Ultrasonic measurement can be used to determine thicknesses of coating materials by transmitting high frequency sound pulses through the material, receiving echo pulses reflected from the substrate surface and interface layers, and measuring the time between pulses. An ultrasonic technique described by U.S. Pat. No. 4,702,931 for Falcoff is limited to measuring the thickness of wet paint. Ultrasonic measurements, such as the technique described in U.S. Pat. No. 5,038,615 to Trulson, et al. for “Ultrasonic Multilayer Paint Thickness Measurement” typically require a liquid couplant, and measurement are limited in thick and coarse grained materials because of the high attenuation of the ultrasonic signal. Ultrasonic techniques typically lack sufficient accuracy for modern applications, such as low observable coatings, lightning strike grids, and metal oxide hull paints.
U.S. Pat. No. 6,120,833 to Bonnebat et al. for “Method And Device For Measuring The Thickness Of An Insulating Coating” describes a method for measuring paint thickness that entails using an inductive measuring technique, along with a second technique, such as an optical or capacitive technique. The method of Bonnebat et al. requires measuring the substrate both before and after coating, and such before-coating measurements are not always available.
Microwaves can be used to penetrate paint and other dielectric coatings to detect corrosion beneath the coating, as described in U.S. Pat. No. 6,674,292 to Bray et al. for “Microwave Corrosion Detection Systems and Methods,” which is hereby incorporated by reference. Bray et al. describes directing relatively low power microwaves toward a coated substrate and measuring the reflected wave to determine the presence of corrosion under the coating. Bray et al. does not each measuring thickness of the coating.
N. Qaddoumi et al. in “Microwave Detection and Depth Determination of Disbonds in Low-Permittivity and Low-Less Thick Sandwich Composites,” Research in Nondestructive Evaluation, 8:51-63 (1996) describes locating defects in a multilayer composite material by directing microwaves toward the material and measuring the phase of the reflected microwave. By varying the frequency and the standoff distance of the microwave transmitter, the phase of the reflected wave can identify characteristics of a defect in a specific layer. Calibration samples are fabricated by creating defects at different locations in each of the layers and measuring the phase of the wave reflected from each of the defects. When measuring an actual product, one can match the phase of the reflected wave to the calibration data to determine which layer, if any, is defective.
U.S. Pat. No. 5,748,003 to Zhoughi for “Microwaves Used for Determining Fatigue and Surface Crack Features on Metal Surface” teaches using microwaves to determine crack geometry under a paint layer. U.S. Pat. No. 5,539,322 to Zhoughi for “Calibrated Microwave Dielectric Coating Thickness Gauge” describes a method using microwaves to determine whether an automobile has been repainted by determining the number of paint coatings that have been applied. By comparing the output voltage of microwave detector with the known output of typical paint layers, the detector can determine the number of paint layers. The method is useful for only a limited number and thickness of coatings, because the measured voltage has a one-to-one correspondence to layer thickness over only a limited range. Because the goal is to determine whether the automobile has been repainted, limited range and accuracy is acceptable.
U.S. Pat. No. 6,005,397 to Zhoughi for “Microwave Thickness Measurement and Apparatus,” describes a method for measuring the thickness of rubber covering steel belts of a tire during a tire retreading process. A series of crystal detectors are mounted along a waveguide to measure a standing wave. By measuring the voltage of multiple points, the amplitude and phase of the standing wave can be calculated. The patent teaches determining the thickness from the standing wave based on a mathematical model, which derives reflected wave amplitude and phase using assumptions about the electrical properties of the coating and the resulting signals. The air gap is preferably selected so that the phase change sign when the rubber layer approaches the desired thickness, thereby alerting operators to stop the buffing process when the phase changes and the desired thickness is reached. The method requires multiple sensors in different physical locations to characterize the standing wave, and the characterization depends on knowing the exact location of the each of the multiple sensors. The assumptions required in this method ultimately limit its accuracy.